rename Fence to Greywall as GreyHaven sandboxing component

Rebrand the project from Fence to Greywall, the sandboxing layer of the
GreyHaven platform. This updates:

- Go module path to gitea.app.monadical.io/monadical/greywall
- Binary name, CLI help text, and all usage examples
- Config paths (~/.config/greywall/greywall.json), env vars (GREYWALL_*)
- Log prefixes ([greywall:*]), temp file prefixes (greywall-*)
- All documentation, scripts, CI workflows, and example files
- README rewritten with GreyHaven branding and Fence attribution

Directory/file renames: cmd/fence → cmd/greywall, pkg/fence → pkg/greywall,
docs/why-fence.md → docs/why-greywall.md, example JSON files, and banner.

docs/recipes/npm-install.md

@@ -18,7 +18,7 @@ Goal: allow npm to fetch packages, but block unexpected egress.
 Run:
 
 ```bash
-fence --settings ./fence.json npm install
+greywall --settings ./greywall.json npm install
 ```
 
 ## Iterate with monitor mode
@@ -26,7 +26,7 @@ fence --settings ./fence.json npm install
 If installs fail, run:
 
 ```bash
-fence -m --settings ./fence.json npm install
+greywall -m --settings ./greywall.json npm install
 ```
 
 Then add the minimum extra domains required for your workflow (private registries, GitHub tarballs, etc.).